I'm afraid as usual he's talking nonsense.
For DOF you may need to multiply the aperture by crop factor to get the FF equivalent. With exposure you don't. Light gathering is not about the total number of photons hitting the sensor, but the brightness of light hitting the sensor. Lens aperture correctly gives the brightness of the lens sensor.

I've used a great many adapted lenses on my µ4/3 cameras. Mainly legacy lenses that were much cheaper than native lenses, but occasionally they've been modern lenses brought for my DSLR.
I've only ever noticed issues with lenses shorter than ~20mm - the extreme telephotos are a real pain to use, but that's just down to the difficulty always apparent in such focal lengths. Not many photographers would expect to find a 1200mm f8 equivalent easy to use, (with care & a solid tripod it's not too bad)

I've used a great many adapted lenses on my µ4/3 cameras. Mainly legacy lenses that were much cheaper than native lenses, but occasionally they've been modern lenses brought for my DSLR.
I've only ever noticed issues with lenses shorter than ~20mm - the extreme telephotos are a real pain to use, but that's just down to the difficulty always apparent in such focal lengths. Not many photographers would expect to find a 1200mm f8 equivalent easy to use, (with care & a solid tripod it's not too bad)

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From an optical point of view, it's hard for me to find an advantage to using any legacy lens shorter than 35mm on M4/3. Those retrofocus SLR lenses are by necessity, much larger, heavier, slower, and more expensive than a short-flange native option with a smaller image circle. And wide open, most will be underperformers optically in terms of sharpness and aberrations like CA. That's not to say they can't produce very pleasing images with a specific look to them, but that's not the ballpark I see adapted lenses working best on M4/3.

I used to use adapted lenses a lot, but now I find the best place to play with them is in the 35-200mm range. This is the region where fast SLR lenses can be found at somewhat reasonable prices, and the crop factor usually helps rather than hinders, bringing you from short-to-moderate telephoto.

As far as longer glass goes, though, you need to spend pretty big money to make a super-telephoto lens worthwhile, at which point the utility of dealing with a heavy, unstabilized, fast manual focus with a 4 degree angle of view starts to get kind of daunting for most people. Super teles under the $200 mark will tend to be some combination of soft wide open, slow, or riddled with chromatic aberration. That said, my "lightweight" Tamron 300/f5.6 probably produces ultimately sharper images than my Panasonic 100-300 despite all that and it's $60 price tag, but that's also when used diligently locked down, prefocussed, on a tripod, and with a remote shutter release. The 100-300 is so easy to use that I'm far more likely to whip around with it handheld despite it's absurd reach, so I'm not engaging in the same level of shot discipline.

ive used many adapted lenses to what was imo very good effect on m4/3, dating back to the old ep2 which i used extensively with schneider kreuznach dkl lenses made for the kodak retina film series of cameras. personally, i much preferred the IQ i got from those lenses than to the native lenses i used at the time. over the years ive also enjoyed using rangefinder lenses from leica, nikon and canon. personally, i like aperture rings and distance/dof scales on my lenses, and i find the IQ of many fits my eye better than many native lenses. the lenses discussed above are also quite compact and fit well ergonomically with m4/3 cameras.

that does not mean i achieved IQ similar in any way to FF. the IQ i did achieve was however often more to my personal liking than most native lenses. what is also spectacularly valid in the age of mirrorless cameras that now exist in all formats--FF, apsc, m4/3 and smaller--is that legacy glass such as i used lasts forever and can be used on all these varied mirrorless offerings, even as the cameras themselves have shorter and shorter shelf lives and we as consumers seem to change systems and formats with similar frequency to that with which we change socks. ):

I don't know anything about this guy, but he fails for me because he provides no explanations for the conclusions he reaches. I do believe DxoMark results in general but do not have the interest level necessary to check out whether his comparative charts and conclusions are valid.

That said, I did read a very interesting and convincing article a few months ago that involved testing of FF vx M43 lenses. The FF lenses did fall down on resolution. The explanation (somewhat vaguely remembered) was that the structure of the M43 sensor included a layer of glass that affected the imaging results, necessitating design considerations that were present in M43 lenses and not (of course) present in FF lenses.

So --- the guy might be at least partially right without his understanding why the results are the way they are.

This is purely engineering curiosity for me. I do very little with adapted lenses and nothing with FF lenses.

I don't know anything about this guy, but he fails for me because he provides no explanations for the conclusions he reaches. I do believe DxoMark results in general but do not have the interest level necessary to check out whether his comparative charts and conclusions are valid.

That said, I did read a very interesting and convincing article a few months ago that involved testing of FF vx M43 lenses. The FF lenses did fall down on resolution. The explanation (somewhat vaguely remembered) was that the structure of the M43 sensor included a layer of glass that affected the imaging results, necessitating design considerations that were present in M43 lenses and not (of course) present in FF lenses.

So --- the guy might be at least partially right without his understanding why the results are the way they are.

This is purely engineering curiosity for me. I do very little with adapted lenses and nothing with FF lenses.

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The article you are referring to is probably the one from the LensRentals Blog, probably the single most authoritative resource for optical measurements on the internet. Far better than DXO, which uses obfuscated methodology, has a highly limited number of test samples (if they ever use more than one at all), and use their somewhat abstracted measurements as a showcase for their software, rather than as a bread & butter requirement of a lens rental agency that needs to deliver reliable quality to its customers.

For what it's worth, the quality degradation due to sensor stack thickness is not a precondition of all FF legacy lenses, because it is dependent on the exit-pupil-to-sensor-plane distance, it primarily affects short focal length rangefinder lenses. An adapted SLR fast 50, 85, or 135 is unlikely to be affected that much, even on a thick-glass sensor. That's especially the case if you're stopping down to f2 or f2.8.

I think the total amount of light matters. That larger objective of a FF lens is gathering more light for the desired image than a smaller m43 lens (or DX lens for that matter). So in a crop sensor you are using just a portion of that total light which may very well be less total light than an equivalent native lens. That's why focal reducers work so well, they re-concentrate that total gathered light onto the smaller sensor. It isn't just math (x .71, etc..), its concentrating all of the light that bigger, oversized for the cropped sensor, lens gathers onto the smaller sensor producing a brighter, richer, cleaner image.

my point above is that we get lost in data and wordplay. its 'FF quality', its 'interplay of lens (this) and sensor (that)’, its 'measuring' resolution and IQ in charts...the truth is its your eyes man, what your eyes tell you. thats what matters. no grand overarching statements, no objective truth applied to subjective art. just trust your eyes. and in this particular case, trust the things that have stood the test of time in countless situations: great glass.

... That larger objective of a FF lens is gathering more light for the desired image than a smaller m43 lens ...

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I'm not sure I'd explain it the same way you did but I think we agree. The f-stop number is really telling us about the density (flux) of photons hitting per square cm. (or your favorite area unit) of the sensor. f2.8 produces the same flux in a lens for 8x10" cameras as in a lens for M43 cameras. The flux that the FF lenses deliver beyond the area of the smaller sensor is not only wasted, it is to a degree bounced around inside the lens and reduces contrast. What the focal reducer is doing is squashing down the FF flux area to make is smaller and hence, because the total number of photons has not changed, it is increasing the photons per unit area on the crop sensor.

This is the key that I think a lot of the explanations really muddy up. The lens is not a bucket, the individual photosites are but this only matters because of the sensitivity and electrical noise of each photosite. The total light metaphor tries to simplify this but in my mind just confuses things, add to this the equivalent aperture nonsense, and we have a real hash of a complex topic.

Yeah, I think the lightbulb moment is when you recognize that what your camera is doing is recording the position of individual, discrete light particles called photons. And the recording of each of those individual photons is sort of a random event. And each one has a measurable impact on the photograph. So if you have a bigger sensor area, you're less likely to notice the individual effects of those random photons, because their contribution to the overall image is smaller.

Most people assume if you put a high quality FF lens on a crop body you will get IQ comparable to a FF body. Not necessarily.

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really ... most people?

I would have doubted *any* people ... I think that most of us that use adapted FF lenses do so because:

its cheap

there isn't (or wasn't) a similar focal length (at F-Stop) in native

its kinda fun

there are unique looks to be had

its cheap

If anyone thought that an adapted FF lens on a smaller format would magically transform their 16Megapixel camera into a 40 Megapixel competitor then they're wishful thinkers (or perhaps not even thinkers).

Err ... you weren't like ... ya knoa, in that group before you saw that video were you?

Sometimes that "Holga" looks interests some (like me){}

Sometimes a 200mm f4 with higher actual transmission isn't available within budget and an adapted 200mm f4 is like $50

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